1. Field of the Invention
This invention relates to drill strings. More specifically, the invention relates to apparatus and methods for controlling vibrations of the drill strings.
2. Description of the Related Art
Various types of drill strings are deployed in a borehole for exploration and production of hydrocarbons. A drill string generally includes drill pipe and a bottom hole assembly. The bottom hole assembly can be used for drilling, sampling, and logging for example.
While deployed in the borehole, the drill string may be subject to a variety of forces or loads. The loads are generally specific to the borehole being drilled. Because the drill string is in the borehole, the loads are unseen and can affect the dynamic behavior of the drill string. For example, the loads may cause the drill string to vibrate. Vibration at a resonant frequency, in particular, can cause severe vibrations at high amplitudes.
An immediate result of the unseen loads may be unknown. If the loads are detrimental, then continued operation of the drill string might cause damage or unreliable operation. In addition, vibrations can limit the lifetime of components in the drill string.
Traditionally, control of the drilling process is performed by a drill operator or driller. The driller relies on his or her experience to control vibrations in the drill string. However, each borehole presents its own challenges and experience may not be able to prevent damaging vibrations in each case.
Therefore, what are needed are techniques for controlling vibrations of a drill string.